To cite the 'PP' package in publications use:

Reif, M. and Steinfeld, J. (2019). PP: Estimation of person parameters for the 1,2,3,4-PL model and the GPCM. R package version 0.6.2. https://github.com/manuelreif/PP

Corresponding BibTeX entry:

  @Manual{,
    title = {PP: Estimation of person parameters for the 1,2,3,4-PL
      model and the GPCM.},
    author = {Manuel Reif and Jan Steinfeld},
    year = {2019},
    note = {R package version 0.6.2},
    url = {https://github.com/manuelreif/PP},
  }